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Senior Member

I definitely agree with Cedra.
You would hear the three forms.
Conditional -- very formal, "appropriate" french. But you wouldn't hear it much, I'm afraid.
Subjunctive -- not so formal. You would hear it in everyday french.
Present -- informal.

Member

Agree with Cedra as well, my solution (present) was maybe too informal.
A bit surprised by your comment Shrekspeare; I would have thought that the subjunctive (au cas où tu dormes) was more formal and less frequently heard than the conditional (au cas où tu dormirais)...

Member

I'm quite sure that you need a conditional here.
- "au cas où" has nothing to do with an order or a necessity (subjunctive).
- To my opinion, the present would not be informal but really incorrect, I'm afraid.